The present invention relates to a method and apparatus for detecting decrease in tire air-pressure and a program for judging decompression of a tire.
An apparatus A in which decompression of the driving wheel is detected on the basis of a correlation between driving force and a difference in wheel speeds on the right and left caused through decompression of either of the right and left driving wheels is conventionally known (see, for instance, Japanese Unexamined Patent Publication No. 2002-59724).
However, such an apparatus A does not consider effects of centrifugal force. It is accordingly difficult to perform accurate judgment of decompression in all speed ranges.
In contrast thereto, there are also known an apparatus B (see, for instance, Japanese Unexamined Patent Publication No. 203678/1997) and an apparatus C (see, for instance, Japanese Unexamined Patent Publication No. 2002-87031) in which decompression is judged by utilizing the fact that a judged value for decompression which is introduced from relative comparison of wheel speed information is correlated to velocity, since a decompressed tire is more strongly affected through centrifugal force when compared to a tire of normal internal pressure, since a decompressed tire is less apt to slip when driving force is applied thereto (the dynamic load radius becomes larger) and since the driving force is correlated to velocity.
In the apparatus B, decompression is judged by utilizing data at the time of deceleration in view of the fact that the slip amount of a decompressed tire decreases when performing driving and that the driving force is correlated to velocity.
However, since only data are accumulated when the vehicle is in a non-driving condition in such an apparatus B, the number of occasions for judging decompression is small, and no attention is being paid to influences of centrifugal force. It is accordingly difficult to perform accurate judgment of decompression in all speed ranges.
On the other hand, while the apparatus C only considers influences of centrifugal force through velocity, this also means that, as a result, influences of driving force are being considered as well, since velocity is correlated to driving torque. In this manner, by incorporating a means for detecting decrease in internal pressure on the basis of relative differences in wheel speed information of tires in the apparatus C, it is possible to perform judgment of decompression of a tire in both of the high speed region and the low speed region.
However, this apparatus C does not consider increases and decreases in wheel torque (running resistance) owing to gradients of sloped roads or accelerations/decelerations of a vehicle. Since the driving force might also change irrespective of the velocity but owing to gradients of road surfaces (sloped roads) or accelerations/decelerations, when decompression is judged on the basis of the correlation with the velocity alone, occasions of performing judgment might become less.
Moreover, since none of the conventional apparatuses A, B and C consider influences of using tires in a mixed manner, erroneous alarm might be issued on the basis of a relative speed ratio (right and left wheel ratio) between right and left tires of the driving wheels which are of different types even though the tires are at normal internal pressure when, for instance, tires of the driving wheels are exchanged to different types of tires with different degrees of wear or front/rear rigidities.